A gene in Lewis rats which causes increased responsiveness of lymphocytes to PHA and Cona A has been transferred to the BN strain. This new congenic strain is histocompatible with BN, but differs markedly in its lymphocyte responsiveness to these mitogens. Experiments are in progress to determine what other effects this gene has, and by what mechanism it operates. The genetic basis of susceptibility to experimental allergic encephalomyelitis is also being explored. The Lewis, Fischer, DA, BN, AVN and LEW.AVN strains are currently being studied.